1. Field of Invention
The present invention relates to a device for regulating one or more of the operative stations of an electrophotographic apparatus to provide more uniform copy output. More specifically, the invention concerns such regulation in electrophotographic apparatus to compensate for sensitometric changes in its photoconductive insulator member occurring during the period of its overall lifespan.
2. Description of Prior Art
It has been observed that under repeated use in electrophotographic apparatus, certain properties of a photoconductive insulator member (herein referred to also merely as a photoconductor) deteriorate because of surface scumming. Specifically, U.S. Pat. No. 3,575,505 describes that the ability of such a photoconductor to discharge in background (light exposed) areas, decreases with repeated use as residual electrographic developer, not removed by cleaning operations, builds up on the photoconductor. To eliminate the adverse effect of this variation in charge dissipation per given exposure, that patent teaches the provision of a compensating variation in the bias of the development electrode located at the toner application station. In the patent it is indicated that the program for such compensation can be keyed to a cumulative copy count or to the cumulative amount of toner usage.
Also, it has been observed heretofore that the electrical properties of photoconductive insulating members become temporarily electrically fatigued after repeated reuse, "electrical fatigue" in this context referring to a change in photo-electrical responses of the member associated with the accumulation of trapped electrons within the volume of the member. Prior art techniques for avoiding the adverse effects of electrical fatigue include (1) exposing the member to an erase illumination, separate from the imaging illumination; (2) heating the member and (3) applying regenerative charge to the photoconductive insulating member of polarity opposite the primary charge. (See for example U.S. Pat. Nos. 2,741,959 and 2,863,767 and Electrophotography by R. M. Schaffert, 2nd Edition, page 87.)
While the prior art techniques for compensating for scumming and electrical fatigue are effective to some extent in achieving more predictable performance from reusable photoconductive insulator members, and hence more uniform copy output, I have discovered that certain photoconductors undergo changes in sensitometric properties during their useful lifespan (e.g., 60,000 cumulative copy-making usages) which are not related to physical effects such as scumming or electrical fatigue. It is hypothesized that these changes are related to changes in the chemical structure of the photoconductors, caused by photochemical decomposition and/or ion bombardment. I have further noted that these changes in sensitometric properties will occur, in a given specie of photoconductor subjected to a repeated reuse in a given apparatus cycle, according to a definite program, which can be determined empirically and which is applicable to all photoconductors of the same type. It has been found advantageous to compensate for such sensitometric changes, either separately or in conjunction with the prior art techniques which compensate for scumming and electrical fatigue, to achieve more uniform copy during the lifespan of the photoconductor.